This invention relates to a method of projecting frames arranged in horizontal and vertical rows on film cards, with a film holder supporting the film card being shifted perpendicular to the beam path and with a lens holder and a lighting unit featuring at least two lenses being shifted at right angles to both the beam path and the direction of movement of the film holder. Furthermore, the invention relates to a microfilm reader designed for carrying out the above method.
When projecting frames, the film holder supporting the film card is normally moved in both coordinate directions, in order to shift the desired frame into the beam path of the lens. This renders it possible to obtain on a projection surface or a screen an image that is free of distortion and that is illuminated with a sufficient degree of uniformity. In order to render the film holder capable of being moved at right angles in relation to the beam path, the unit must, however, have a relatively large design width, which is undesired in many cases.
In order to avoid a large design width, it is common to shift the lens together with the lighting unit at right angles in relation to the beam path instead of the film holder. In this case, the lighting unit is rigidly coupled with a lens holder with which it synchronously follows the movement of the lens.
In this method the illumination of the projected frame is, however, not sufficiently uniform for many applications, and considerable difficulties occur when lenses having different focal lengths are to be used alternatively. With microfilm readers having a plurality of lenses, the latter are normally mounted pivotably, so that the lens desired for one given application can be swivelled into alignment in relation to the lighting unit.
According to U.S. Pat. No. 3,963,337, it has already become known for still projectors to shift the frame to be transilluminated parallel to the optical axis of the lens, so as to obtain an image free of distortion, with the image being above the optical axis of the lens. The lighting unit is mounted pivotably, in order to obtain uniform illumination. This arrangement of the lighting unit necessitates substantial design expenditure and has to date not been considered for microfilm readers.